1. Field of the Invention
The present invention relates to an optical fiber amplifier, and particularly, to an optical fiber amplifier for amplifying optical signals using a pump light.
2. Description of the Related Art
Optical amplifiers for directly amplifying optical signals are classified into (a) optical fiber amplifiers employing an optical fiber doped with rare earth elements such as Er (erbium), Nd (neodymium), or Pr (praseodymium), (b) semiconductor laser amplifiers doped with rare earth elements, and (c) optical amplifiers using a nonlinear effect in an optical fiber such as a stimulated Raman scattering effect or a stimulated Brillouin scattering effect.
Among these optical amplifiers, the optical fiber amplifiers (a) have no polarization dependency, cause low noise, and involve low connection loss with respect to a transmission optical fiber. These optical fiber amplifiers are used to extend the relay distance and the number of branches of the optical signals in an optical communication system.
In particular, the optical fiber amplifiers employing the Er-doped optical fiber are capable of amplifying optical signals in the 1.5-micrometer band that is not attenuated in an optical fiber. Accordingly, optical fiber amplifiers employing Er-doped optical fiber are more practical than those employing Nd-doped optical fiber for amplifying optical signals in the 1.06- or 1.3-micrometer bands or Pr-doped optical fiber for amplifying optical signals in the 1.3-micrometer band.
The rare-earth-element-doped fiber in the optical fiber amplifiers amplifies optical signals using pump light, which is applied according to a forward pump method, a backward pump method, or a bidirectional pump method. These methods differ from one another in the propagating direction of the pump light with respect to the optical signal.
The Er in the Er-doped fiber demonstrates absorption and fluorescence characteristics, so that the gain of the amplifier employing the Er-doped fiber is dependent on the wavelength of the optical signal light. The gain reaches peaks at 1.536 and 1.55 micrometers in the wavelengths of the optical signal light. The gain drops at around 1.52 micrometers in the wavelength of the optical signal light. The optical fiber amplifiers for amplifying the optical signal light in the 1.5-micrometer band use pump light of 1.48 or 0.98 micrometers wavelength.
To compensate for the loss of gain in the Er-doped optical fiber amplifiers near the 1.54 micrometer wavelength of the optical signal light, the optical fiber may be doped with Er and Al (aluminum). The Er-Al-doped optical fiber exhibits less fluorescence at a wavelength of 1.53 micrometers. The Er distributed in the core below the threshold of the pump light gives no amplification and causes an absorption loss in the optical signals around a wavelength of 1.536 micrometers. This flattens the gain/frequency curve of the amplifier.
To amplify optical signals having a wavelength of 1.55 micrometers, the Er-doped fiber must receive pump light having a wavelength of 1.48 or 0.98 micrometers. The pump light of 0.98 micrometers achieves a good gain and can be handled by simple optical couplers. The wavelength of this pump light, however, greatly differs from that of the optical signal light, so that the field diameter (a width at 1/e.sup.2 of a maximum value on a luminous intensity curve) of the optical signal light greatly differs from that of the pump light. Accordingly, even the Er-Al-doped optical fiber is insufficient to flatten the gain/frequency curve of the amplifier.
The optical fiber amplifiers using rare-earth-element-doped fiber are applicable to a wavelength multiplexing system. In this case, filter elements must be disposed in the vicinity of the output ends of the amplifiers, to provide a uniform gain over a range of more than ten nanometers on the signal light gain/frequency curve.
A single wavelength system also needs filter elements to prevent oscillations due to spontaneous emission through many optical amplifiers.
However, using the filter elements and other optical parts increases the cost and decreases the reliability, of the system.